She could have eaten something toxic or swallowed something she shouldn't have. I am no expert on chicken illnesses, so I am only telling you what I would do if she were my bird.. And if she is not eating or drinking, the suggestions I have would need to be syringed down her throat. But you have to do this VERY slowly so you don't aspirate her. So you would DRIBBLE this down her beak with an empty syringe. Even if half of it never gets down her beak, it must be done very very slowly.
You can do a molasses or an epsom drench to flush out toxins. The molasses is mixed 1/4 cup molasses to 1 gallon of water. The epsom is mixed 1 teaspoon epsom salt to 1 cup of warm water. If she is not drinking, one of these can be put down the beak once an hour till she really starts pooping. Pick one or the other.
If she is eating, you can take some warm mixed up oatmeal and if you have any charcoal capsules, you would mix in 4 capsules. (open each one up and pour contents in).
Any of these will flush something toxic out of a chickens body.
I doubt at her age she needs to be wormed. And if this is a VERY sudden thing, I doubt it is a bacterial infection. Usually bacterias wear at the bird for a long time. It still could be Mareks or possibly she got into some moldy feed. Rotten or moldy feed will cause paralysis.
I have no other ideas for your. If she is cold, (feel the feet) then get her under a heat lamp. Cold birds can die much faster than warm birds. Cold birds will not eat when sick. So get her warm. This is about all I can offer you. Keep her comfortable and try the drenches on her IF you think she may have eaten something making her sick.
Also, and again, do an egg check. Egg binding can cause all sorts of symptoms too.
Keep us posted and I hope she pulls thru.